1. Field of the Invention
The present invention relates to an optical transmission fiber system. More particularly, it relates to an optical transmission system for optimally compensating a wavelength dispersion characteristic of a single mode optical transmission fiber.
2. Description of the Related Art
High-capacity transmission systems are strongly expected in future multi media network. Several schemes have been proposed to realize a super high capacity optical transmission system, for example, Time-Division Multiplexing (TDM) system, an Optical Time-Division Multiplexing (OTDM) system, which is used in optical areas, and a wavelength-Division-Multiplexing (WDM) system.
An Erbium Doped Fiber amplifier (EDFA) having a wide gain band in the 1.5 .mu.m band has been put to practical use as a pre-amplifier, a post amplifier, or a repeater amplifier. The WDM system among the above-described various kinds of systems is more useful as a system for obtaining a flexible optical wave network, in which cross-connecting, branching, inserting optical signals, and multiplexing different kinds of services are performed.
Further, the WDM system is more preferable to other systems in the case where a network constructed by using a 1.3 .mu.m zero-dispersion single mode fiber, which is called as SMF, is employed as high-capacity repeaterless transmission system. This is because that as the transmission speed per optical carrier signal is lowered, it maybe possible to make acceptable values large, of wavelength dispersion and input optical power which is limited due to nonlinear effect of an optical fiber.
However, transmission speed and distance are limited because a transmission waveform is distorted due to a wavelength dispersion characteristic or interaction of the characteristic and nonlinear effect, in the case where an optical signal having a high speed more than 10 Gb/s per carrier wave is transmitted.
The wavelength dispersion characteristic means a characteristic, in which speed is changed for each wavelength according to a refractive index having wavelength dependency. Therefore, a delay amount becomes different per carrier wavelength, and then waveform distortion is generated.
It is general to use a system for inserting a dispersion compensating device having dispersion, of which symbol is reversed to that of transmission fiber dispersion generated in a transmission fiber, and offsetting the transmission fiber dispersion as a system for suppressing the wavelength dispersion. Several systems, for example (1) a system employing a grating technique, (2) a system employing an optical interferometer, or (3) a system employing an optical fiber have been already proposed, as dispersion compensating device.
Further, a system for performing pre-chirping in a transmitter, that is, a system for intentionally modulating phases or frequencies to suppress dispersion modulating phases or frequencies to suppress dispersion due to wavelength dispersion other than intensity signal components which is modulated with a base band signal, or a system for shaping waveform by a pulse compression effect according to nonlinear effect in SMF, has been proposed.
A dispersion compensating fiber (DCF) is most attractive among various kinds of dispersion compensating devices from the view point of controlling operation in a wide band and dispersion, and stability.
FIG. 1 shows a structural example of a general wavelength division multiplexing (WDM) transmission system having a plurality of repeaters, which employs a SMF transmission line.
A system shown in FIG. 1 performs wavelength division multiplexing transmission under the condition of 3 repeaters, 16-carriers.times.4=280 Km.
Further, in FIG. 1, there are 16 pieces of laser diodes 1 (LD) corresponding to 16 carriers.
A Lithium Niobate (LiNbO.sub.3) external modulator, which is so-called as LN, emits different carrier wavelengths, each modulated corresponding to a signal.
An optical coupler 3 receives optical outputs of the emission spectrums, each of 16 different wavelengths respectively sent from the 16 pieces of laser diodes (LD) 1, combines and outputs them. A branch circuit 4 having a branching ratio of 10:1 branches and outputs 1/10 of the combined signals outputted from optical coupler 3 and inputs it to a wavelength stabilizer 5.
The wavelength stabilizer 5 feeds a control signal back to a driver circuit of each of laser diodes (LD) 1 so as to set carrier wavelength signal to a predetermined wavelength.
Further, in the above-described external modulator 2 and the optical coupler 3, attenuation is generated. Accordingly, a post amplifier 6 is provided to amplitude an emitted level to the original level. The post amplifier 6 is formed of the above-described Elbium Doped Optical Fiber.
A reference numeral 7, for example, it is 1.3 .mu.m zero-dispersion single mode fiber, that is SMF, has a length of about 70 Km.
A branch circuit 9 receives the optical signal sent from the optical transmission line, and the optical signal branched by the branch circuit 9 is inputted to an optical to electrical convertor 12 via a band pass filter 11 having a central wavelength of 0.3 nm to covert it to a corresponding electrical signal.
In FIG. 1, a dispersion compensating circuit 8 is inserted on the way of the optical transmission line. Dispersion compensating circuit 8 is formed of a dispersion compensating fiber (DCF) 80, a pre-amplifier 81 and a post amplifier 82.
In FIG. 1, when one wavelength is transmitted, only one laser diode (LD) 1 may be employed and no optical coupler and optical selective filter are necessary.
Further, dispersion compensating fiber (DCF) 80 has a larger dispersion absolute value, and symbols of the value are reversed to the distributed value of SMF 7 to give dispersion compensation.
Dispersion compensating fiber (DCF) 80 is employed to give dispersion compensation. However, a core size of the fiber 80 is substantially small, and a nonlinear refractive index is large. That brings great nonlinear effect in comparison with that of a normal SMF or a dispersion shifted fiber.
In this case, nonlinear effect that appears extreme is self phase modulation (SPM) when a signal source outputs one wave carrier for TDM and OTDM, while it is cross phase modulation (XPM) as well as self phase modulation (SPM) when the signal source outputs multi-wave carriers.
In either cases, wavelength chirping is generated on the optical signals, and transmission waveforms are distorted resulting in deterioration of the transmission characteristic. It is the easiest way for reducing the nonlinear effect in dispersion compensating fiber (DCF) 80 to make input power of the DCF 80 lowered.
However, if the input power of DCF 80 is lowered, SN ratio becomes worse according to a loss. Or the input power of SMF 7 should be increased, so that the nonlinear effect in SMF 7 appears extreme.
In either way, there are limitations in an allowed range of input power, transmission distance and transmission speed in comparison with the case where there is no nonlinear effect in dispersion compensating fiber (DCF) 80. That brings some problems, such as optical signal transmission via the SMF becomes difficult.
Further, even by various kinds of the above described dispersion compensating systems, such as the system for pre-chirping in a transmitter or the system for compressing pulses and shaping waveform of pulses with the non-linear effect in a SMF, there has been a problem that tolerance of input power to SMF 7, in which shaping of waveform may be effectively performed, is narrow, if only one of the above described systems is employed.
When transmitting 1-wave, it is possible to control the output of the optical amplifier so as to be an input power to SMF 7, of which waveform can be shaped.
However, when employing the WDM transmission system, the problems appear as follows;
(1) It is difficult to constantly keep output power of each channel because of dispersion of the optical output power in an optical source or losses in optical devices; and PA0 (2) The dispersion is generated in optical outputs between channels, because there is a gain tilting on amplifying characteristic of an optical amplifier, so that a different wavelength makes different amplifying power. Therefore, it is difficult to stably transmit WDM signals on a SMF transmission line.